Successfully installing car audio components into a vehicle can be difficult given the fact that the chassis of the car is used as a ground return for virtually all of the car's electrical accessories. The car's alternator and electrical accessories cause large changes in the magnetic fields surrounding the accessory power and control leads as well as the car's chassis. As a signal is routed from component to component, noise can be introduced into the system due to the proximity of the signal route to changing electro-magnetic fields. Also, noise can be introduced into components via the power supply or internal signal paths. One method of handling a signal in hostile noisy environments is to increase the level of the signal so that the noise will be small in comparison, i.e., increase the S/N ratio by increasing the signal level.
To be effective, an increase in the level of the signal must be at the source. Increasing the signal level after the source would also increase the noise floor. In car audio, the source is commonly called the deck or headpiece. Decks usually have four signal outputs for feeding a 4-speaker stereo system in the car; the name assigned to the output describes the speaker position within the car, namely: for the left channel (LC), left front (LF), and left rear (LR); for the right channel (RC), right front (RF) and right rear (RR). The maximum signal level output of most decks is approximately 2.0 volts rms (root mean square)--approximately 5.6 volts peak-to-peak. This is because virtually all modern vehicles operate on a nominal 12.0 volts DC and therefore the circuitry in virtually all modern car audio decks also operates on a nominal 12.0 volts DC. Once this 12.0 volt level is regulated and connected to the audio circuitry in a deck, the highest signal level attainable is only around 2.0 volts. This is because of voltage drops within the semiconductor devices of the deck. So if the audio circuitry within the deck is operating on 12.0 volts DC, the signal level will be around 2.0 volts rms.
One popular technique to increase the signal output level is to convert the car's nominal 12.0 volts DC into higher levels such as 20 volts or even 30 volts so that the pre-amp level output stages can accommodate higher signal levels. The power supply convertors necessary to increase the 12.0 volts, however, makes these decks expensive.